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HR Wallingford, Wallingford, UKPublished in the proceedings of River Flow 2012, 5-7 September 2012

AbstractReservoir sedimentation is a main concern in the Tarbela reservoir in Pakistan. This major storage reservoiron the Indus River, constructed between 1968 and 1974, plays a key role in the provision of water forirrigation, power generation and flood control. Sediments have reduced 30% the initial capacity of thereservoir (11,600Mm3). The advance of the foreset slope towards the dam also increases the risk of blockingthe low level outlets that provide flows downstream to the irrigation system and to the power station.The paper presents historical data of the evolution of the sediment deposits in the reservoir and how thisdata has been used to validate a numerical model, RESSASS, that predicts the future development of thedelta. The advance of the delta is clear when analysing the surveyed longitudinal profiles and the numericalmodel is able to predict very accurately this behaviour. Several aspects of the analysis of the future evolutionof sediment deposits are discussed including the influence of upstream reservoirs that could reduce theincoming sediment towards Tarbela and the need to estimate the likely amounts of sediment passingthrough the turbines.1. IntroductionTarbela Dam was constructed in the 1970’s on the Indus river in north central Pakistan. It was conceived tohelp to regulate the seasonal flows both for irrigation of the Indus plains downstream and for generation ofhydropower. Tarbela is a strategic national resource providing 50% of the total irrigation releases and 30% ofthe total power and energy needs of Pakistan.Tarbela Project comprises three dams, the main embankment with a length of 2,750m and a height of 143m.The reservoir had an initial capacity of 11,600Mm3and a reservoir length extending approximately 70kmupstream the dam.The Indus River carries a very high sediment load. This is largely due to the erosive effect of the glaciers thatsupply much of the flow. It is estimated that over 200 million tonnes of suspended sand, silt and washload(Lowe and Fox, 1982) are deposited entirely in the reservoir accumulating in the form of a delta that growstoward the dam. When the project was conceived it was considered that Tarbela Reservoir would have beenfilled with sediment within 30 years but sediment rates have been lower than expected.

Tarbela Dam in Pakistan.

Case study of reservoir sedimentation

M. Roca

HRPP533

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Figure1: Catchment of the Indus River at Tarbela Dam. Catchment area subject to monsoon rainfallshadowed.

However, sedimentation at Tarbela Reservoir has been a concern for a number of years. The trap efficiencyof the reservoir is high because its shape leads to deposition of almost all the incoming sediment. Thereduction in live storage since 1974 has been estimated in 2009 as 30%. The decrease in live storage is aconcern as it may result in reduction of irrigation releases and power supply. The impact of the delta createdby the sediment deposits approaching the main dam may also cause problems clogging the intakes feedingthe turbines. The instability of

the downstream sloping face of the delta may result in sloughing or landslides(Lowe and Fox, 1982). The occurrence of an earthquake may give raise to larger landslides (TAMS, 1998).This paper presents historical data of the evolution of the sediment deposits in the reservoir. Chapter 2discusses the existing available information about water and sediment discharges, longitudinal profiles andcross-sections at the reservoir. The characteristics, validation and application of a specific reservoirnumerical model, RESSASS, is described in chapter 3. The model predicts the future development of thedelta and estimates the amount of sediment passing through the intakes at the dam. The capabilities andlimitations of the numerical simulations are also discussed.2. Existing data2.1. Water dischargeThe Indus basin upstream of Tarbela Dam has an area of 169,650 km2. Over 90% lies between the GreatKarakoram and the Himalayan ranges and meltwaters from this region contribute to the major part of theannual flow reaching Tarbela. The remainder of the basin, lying immediately upstream of the dam (Figure 1),is subject to monsoon rainfall primarily during the months of July, August and September. The monsoonrains runoff causes sharp floods of short duration which are superimposed on the slower respondingsnowmelt runoff.

Tarbela Dam in Pakistan.

Case study of reservoir sedimentation

M. Roca

HRPP533

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Figure 2: Water inflows to Tarbela Dam at different years.The average annual inflow to Tarbela is 81,000 Mcm (TAMS 1998). As the Indus has a high proportion ofsnowmelt runoff the variability of the annual runoff is relatively modest. Peak flows due to snowmelt can beas high as 5,660 to 11,300 m3/s with an additional rainfall contribution typically reaching a maximum of 5,660m3/s (Figure 2).Water discharges are measured at Besham Qila (Figure 1), a gauging station located on the Indus riversome 60 km upstream of the top end of the reservoir. It provides valuable information on water dischargesand sediment concentrations. About 93% of the inflows to Tarbela dam originate upstream of Besham Qila.The existing information consists on a set of water discharges and levels at the dam every 10 days from1968 to 2009. A more detailed data set, with daily records, is available from the period 1993 to 2010.2.2. Sediment loadInformation on sediment inflows into Tarbela reservoir has been collected from several sources. TAMS(1998) shows that annual sediment inflows vary between 100 and 300 MT with an average just below200 MT for the period 1967-1996. Lowe and Fox (1984) also state that the average sediment inflow inTarbela is 200 MT, 97% or more carried during the large flows in summer, between May and September witha peak in July and August, primarily caused by snowmelt. White (2001) states a value of 240 MT, with 40MTof very fine sediment passing down- stream the dam. Tarbela Dam Project (2009) provides two estimationsof the average annual sediment yield for the period 1975-2009. The first estimation is obtained by using acomprehensive set of sediment rating curves based on data at Besham Qila and gives a value of 160 MT. Asecond estimatation is based on information from the hydrographic survey showing an annual average of168 MT, excluding the Siran and Brandu loads (tributaries into the Tarbela Reservoir).These differences show the difficulties in providing accurate sediment yield estimations that originate fromhigh spatial and temporal variability of quantities related to the physical processes.

Tarbela Dam in Pakistan.

Case study of reservoir sedimentation

M. Roca

HRPP533

4

Figure 3: Annual average percentage of sand, silt and clay at Besham Qila for the period 2000-2009.One third of the inflowing sediment at Tarbela Reservoir is sand, and the remaining two thirds is silt and clay(TAMS, 1998). Individual samples of suspended concentrations at Besham Qila (the gauging stationupstream the reservoir) show high variation for the sand proportion, from less than 10 % to 75 %. Annualaverage values for the 2000-2009 period (extracted from Tarbela Dam Project, 2009) show a variation ofproportion of sand between 20 and 35% (Figure 3).2.3. Operation levelsSeasonal fluctuations in river discharge together with requirements for irrigation, hydroelectric powergeneration and considerations for safe operation of the reservoir during flood season combine to establishthe operating levels for filling and emptying the reservoir.The operating levels at the dam follow a drawdown and fill cycle: water levels drop after September,reaching a minimum around the months of March, April, May, and rising again during the summer.Operating levels have a great influence in determining the advance of the delta towards the dam. When thewater levels at the dam are low, the sediments deposited in the upper reaches are reworked andtransported downstream within the reservoir. This influence is represented in Figure 4 that shows theadvance of the delta towards the dam as a function of the differences between minimum water operatinglevels and topset bed elevations. Two sets of data are presented in that figure: TAMS (1998) and TarbelaDam Project (2009).

Tarbela Dam in Pakistan.

Case study of reservoir sedimentation

M. Roca

HRPP533

5

Figure 4: Advance of the delta towards the dam2.4. Bathymetry of the reservoirCross-section lines across the reservoir are surveyed every year to compute the volume changes. Some ofthis information has been digitized and analyzed to provide a description of the evolution of bed levels in thereservoir (Figure 5). The longitudinal profile in the reservoir presents a clear delta that has been advancingevery year. In 2009 the position of the delta point was located 10 km upstream of the dam.Tarbela Dam Project (2009) records 917 m of advance of the delta in one year (2009). It has to be noted thatsome inaccuracies are possible in those

estimates because of the difficulties in determining the exactposition of the topset delta point. The density of points in the profile also influences the accuracy of theresults. Due to these difficulties a point at half of the downstream sloping face of the delta, at elevation396.2 m, is used to illustrate the rate of advance of the delta towards the dam (Figure 6). The advance of thedelta slope towards the dam has always been increasing, although at different rates.Bed levels at the dam are important to assess the risk of clogging the intakes. Figure 7 shows bed levels atthe dam at different years. The accuracy of the data varies as some values are obtained from surveys (2008and 2009), from plots included in other reports (1979-1994) or have been extrapolated from surveyinformation at a range line located about 700 m upstream the dam (1998-2006).Bed levels near the dam are higher every year showing an increasing tendency in the last 5 years (Figure 7).

Tarbela Dam in Pakistan.

Case study of reservoir sedimentation

M. Roca

HRPP533

6

Figure 5: Longitudinal bed profile

Figure 6: Advance of the delta towards the dam

Figure 7: Bed levels at the dam

Tarbela Dam in Pakistan.

Case study of reservoir sedimentation

M. Roca

HRPP533

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2.5. Influence of upstream reservoirsA proposal exists to construct a dam at Basha, upstream of Besham Qila, with an approximate catchmentarea of 152,100 km2. It is expected that the dam will be commissioned in 2020. Basha Dam is going to trapsediments of the upper catchments of the Indus, therefore reducing the incoming sediment to Tarbela.Diamer Basha Consultants (2007) reports a trapping efficiency of Basha reservoir around 94% for the firstyears. This amount is expected to reduce to 80%. This efficiency is translated into a reduction of 69% of theincoming sediment to Tarbela.Once Basha is commissioned a new large capacity storage will become available upstream of Tarbela andthe flow sequences through Tarbela will change. The dams will operate in series and the scenarios for waterlevels variations through the year will undoubtedly change.3. Numerical simulation of sedimentation processes3.1. Description of RESSASSThe numerical model RESSASS is used to predict the future sedimentation patterns at the reservoir.The RESSASS model is based upon physically based equations that describe flow and sediment movementin open channels based on steady state backwater computations, and sediment transport calculations for arange of sediment sizes. A time stepping model is used in which initial conditions are input to equationswhich predict water levels and bed levels a short time later. These predictions provide the input conditionsfor the next time step. The cycle is repeated many times to make predictions over the simulation time periodthat can extend over periods of several years with a time step of one day. The flow and sediment transportsimulations are one-dimensional, that is only variations along the length of the reservoir are considered, andall the quantities calculated are averaged over a cross-section.The model requires a time series of the discharges entering the reservoir as input. If the water levelvariations at the dam are not specified they are calculated using a storage routing method. Velocities anddepths through the reservoir are calculated using a backwater computation. The effects of the shape of thereservoir in generating turbulence and mixing are accounted for by adding a term to the shear velocitycalculated from a friction relationship derived from 3-D turbulence modeling.Sediments are divided into different size ranges. The transporting capacities for the sand and larger sizesare calculated separately from finer sediments, silts and clays, in the cohesive size range. Corrections areapplied to both sand and silt concentrations to allow for non equilibrium transport conditions.The sediment masses deposited or eroded at each section are converted to volumes taking consolidationeffects into account. The distribution of sediment deposits across the reservoir sections is varied accordingto user-defined functions. An important aspect of the model is that it calculates the composition of thesediments on the bed of the reservoir from the deposition that has taken place during the simulation. Thusthe sediment sizes of the deposited sediment are predicted, rather than being specified initially, as is thecase when most "river" models are applied to reservoirs.

Tarbela Dam in Pakistan.

Case study of reservoir sedimentation

M. Roca

HRPP533

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3.2. Application of RESSASS to Tarbela damTAMS (1998) calibrates the numerical model RESSASS at Tarbela reservoir comparing measured andestimated loss of storage in the reservoir, observed changes in the longitudinal bed levels and in cross-section geometry for the period 1974-1997. Among the different parameters considered in the calibration,sediment sizes of sand and silt were fixed to provide the better estimation of the observed values. The modelwas validated with the new available information from the period 1997 to 2009 (HR Wallingford 2011). Thenumerical results of variables such as deposited material and transport rates, show that RESSASS describesthe physical processes in the reservoir reasonably well. The comparison between numerical results and realdata shows very good agreement as in the estimation of the rate of advance of the delta shown in Figure 6.3.3. Results of the numerical modelWater inflows and reservoir operation (water level and outflow) both influence the sediment dynamics in thereservoir. As the future water inflows and levels at the dam are not known, several scenarios need to besimulated to establish the uncertainties of the predictions of the future behavior.It has been observed that the future operation levels have a bigger impact than the water discharge timeseries. In this paper two possible scenarios are analyzed: in Scenario 1 the water level rises to maximum asthe river inflow starts to rise. In Scenario 2 it remains low almost till the peak flow (Figure 8).In Scenario 1, the delta arrives near the dam around 2040 which causes less sediment inflow to pass thedam section as well as into the intakes. However, the trapping efficiency is high which decreases the storageof the reservoir. Scenario 2 shows the opposite result. The numerical model predicts a quick advance of thesediment delta between 2010 and 2020 with bed levels at the dam rising rapidly. This indicates thatsediment will pass through the intakes in increasing quantities but preserving the storage capacity. Underthis scenario it is likely to be a maintenance commitment of increasing severity in the long term. TAMS(1998) already stated the need to adopt a suitable reservoir operation policy to avoid sediment ingress to theirrigation and power station intakes.

Figure 8: Comparison of future operation levels

Tarbela Dam inPakistan.

Case study of reservoir sedimentation

M. Roca

HRPP533

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Some parameters are not well simulated by the numerical model such as bed levels at the dam. In the period2005-2009 bed levels calculated with RESSASS do not follow the increase showed in surveys (Figure 7).One possible explanation could be related to the fact that in 2005 a major earth-quake, of 7.6, affected thenorth area of Pakistan, including the Tarbela reservoir. It could have been possible that sediment inflowentering the reservoir had increased due to mobilization of material in upstream catchments, especially finematerial. Some kind of movement or landslides of the sloping faces may also have happened. TheRESSASS model is unable to predict the slumping of material that may happen during an earthquake. It hasnot been possible to review the sediment loads at Besham Qila in that period to study the possible impact ofthe earthquake.When predicting the future storage capacities and bed levels along the reservoir there is the assumption thatthe power station and irrigation intakes can continue to function when the bed levels adjacent to the damexceeds the level of the intakes by quite a large margin.RESSASS also estimates the daily concentrations of sediment arriving at the dam. Combining these valueswith water discharges through the different intakes, the amount of sediment flowing into each intake andthus, through the turbines can be calculated. This is a useful result to estimate the life span of the turbines.Due to the number of assumptions about the different parameters such as the incoming flows, the resultsshould be regarded as an indication of the likely order of magnitude rather than an absolute value.Taking into account that the average annual inflow of sediment into the reservoir considered in the numericalmodel is about 195 MT, the total outflow downstream the dam is calculated as 60 MT per year before thearrival of the delta and about double (or slightly more) after the arrival of the delta, especially of sandmaterial.After 2020, when Basha Dam upstream Tarbela is expected to be commissioned, the amount of sedimentinflow decreases and therefore lowest deposition rates, lowest amounts of silt arriving to the dam and largerstorage volumes are observed. Very low or no proportion of sand material is expected to pass through theintakes after the commissioning of Basha Dam.The numerical simulations taking into account the influence of Basha Dam must be regarded as onlyindicative because residual flows to Tarbela and level sequences within Tarbela are as yet undefined andboth these affect sedimentation quantities and patterns.4. ConclusionsThe information provided by Besham Qila, a gauging station upstream the reservoir, about water dischargesand sediment concentrations and the information obtained from the annual surveys is fundamental tounderstand the sedimentation processes in the Tarbela Reservoir. Field data is also extremely useful tovalidate numerical models. It should be noted that monitoring of sediment related processes is a demandingtask, often associated with a certain degree of uncertainty due to high spatial and temporal variability. Forexample, different estimations of the same variable can be found based on different real observations.The application of a numerical model to simulate sedimentation in the future needs to consider of a series ofwater discharges and operation levels representative of the future scenario. The results will depend on theassumptions made about the temporal series. Short term predictions will depend in part on how close thereal sequences are to the one used as input in the numerical model. It is assumed that predictions for dates10 years or more into the future are representative provided that there is no change in the long term averageinflows of water and sediment and the reservoir is operated in the same manner as the model assumes.

Tarbela Dam in Pakistan.

Case study of reservoir sedimentation

M. Roca

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However uncertainties in the predictions increase with time due to the numerous assumptions made duringthe study not materializing in the future.The longitudinal profile in the reservoir presents a clear delta that has been advancing every year.RESSASS, a 1D reservoir model, has proved to describe well the behavior of the reservoir. The modeledresults match with the estimations based on observations within the expected tolerances. Some limitationsare observed when estimating bed levels near the dam.The sediment deposition in the reservoir and the amounts of material flowing through the intakes aresignificantly influenced by the water level operation, which is given to the model as input data.5. ReferencesDiamer Basha Consultants. 2007. Review of feasibility report, engineering design, tenderdrawings/documents of Diamer Basha Dam Project. Reservoir Operation and Sediment Transport. Mainreport. Volume 1HR Wallingford. 2011. Tarbela 4th extension. Sedimentation study. HR Wallingford report EX6486Lowe, J. & Fox, I.H.R. 1982. Sedimentation in Tarbela reservoir. In Commission Internationale des GrandesBarrages. Quatorzieme Congres des Grands Barrages, Rio de JaneiroTAMS and HR Wallingford. 1998. Tarbela Dam Sediment Management Study”. Main report. Volume 2Tarbela Dam Project. 2009. Annual Survey and Hydrology Tarbela Dam. WAPDAWhite, R. 2001. Evacuation of sediments from reservoir. Thomas Teldford Publishing